The present invention relates to aqueous dispersions of fluoropolymers usable for coating of metal and ceramic surfaces and in the impregnation of textiles, glass, carbon or aramid fibers.
Specifically, the present invention relates to aqueous dispersions of fluoropolymers having an improved stability to segregation, and a lower viscosity variation with the temperature, wherefore they are particularly suitable in the impregnation of textiles and in the obtainment on metal substrata of coatings having improved properties without colouring problems.
It is known that from the polymerization in emulsion aqueous dispersions of fluoropolymers having a dry polymer content generally not higher than 30-35% by weight are obtained. In polymerization a fluorinated ionic stabilizer is generally used, so as to avoid the polymer particle coagulation. To obtain dispersions having colloidal stability, it is necessary that the dry polymer amount is not higher than 30-35% by weight.
However, for coating of various types of substratum such as for example metals, ceramics, or for the impregnation of glass, carbon or aramid fibers, the use of dispersions of fluoropolymers having a higher dry polymer content is advantageous, in particular a dry polymer content equal to 60% by weight. Said dispersions having a high polymer content show the best application characteristics, i.e. a good balance between the deposit of a sufficient amount of material on the substratum, a lower amount of water to be evaporated during the drying phase, and a suitable viscosity of the dispersion in connection with the application technique and the manufactured article to be coated.
Therefore, the dispersions obtained from the polymerization are subjected to a concentration step in which they are stabilized by using suitable surfactants, generally non ionic. The use of said surfactants allows to prepare dispersions having a dry polymer content up to 70% by weight. For the dispersion concentration, various processes are known which comprise evaporation under vacuum (U.S. Pat. No. 3,316,201), electrodecantation (GB-A 642,025), ultrafiltration (U.S. Pat. No. 4,369,266). Alternatively, the reversible flocculation of the dispersion containing the non ionic surfactant is induced by heating over the cloud point temperature of the surfactant (U.S. Pat. No. 3,037,953).
The concentrated dispersion must have very good stability characteristics to segregation to avoid drawbacks during the various working steps (transport, storage, formulation, application) and to allow a sufficiently long shelf-life of the compound. It is known indeed that, due to the difference of density betwen fluoropolymers (density about 2 g/ml) and water (density 1 g/ml), segregation phenomena can easily occur and give rise to the formation of not redispersible aggregates (caking). This impairs the possibility to use concentrated dispersions. Besides, concentrated dispersion must have good wettability properties of the surfaces to be coated and a small variation of viscosity in the application temperature range, so as to allow a careful control of the coating process. In this way during the application of subsequent layers of fluoropolymer, defect formation problems like coagulum, cissing are avoided, which represent the main application limits of these dispersions, especially as regards high productivity application techniques such as impregnation, casting.
A method for stabilizing concentrated dispersions of fluoropolymers consists in the use of non ionic surfactants of the class of alkylphenols or polyethoxylated alcohols. For example U.S. Pat. No. 3,301,807 describes the concentration and stabilization of PTFE dispersions, by the use of surfactants of the class of polyethoxylated alkylphenols. The use of these stabilizers shows various drawbacks related first of all to the decomposition of the surfactant during the fluoropolymer sintering phase. The residues of the surfactant decomposition are indeed responsible of the colouring of the manufactured articles treated with the above concentrated PTFE dispersions. Besides, by using poly-ethoxylated alkylphenols as stabilizers, during the application of the dispersion, it may happen the formation of smokes containing harmful volatile substances.
In U.S. Pat. No. 5,985,966 an aqueous dispersion of fluoropolymers stabilized with a phenolic surfactant is described, whose hydrophilic part, besides the polyoxyethylene chain, contains an ionic group xe2x80x94SO3M (Mxe2x95x90Na, K or NH4). The so stabilized dispersion shows good shear stability which make it suitable to the use for the surface coating, also by multilayer deposition. However the dispersions described in this patent do not allow to obtain the required colour and transparency characteristics of the deposited film. In fact, the surfactant decomposition during the fluoropolymer sintering is responsible of the colouring of the coated manufactured articles.
In U.S. Pat. No. 5,219,910 the use of oxidized alkylamines as stabilizers for obtaining concentrated dispersions of fluoropolymers is reported, able to generate films of fluorinated material having improved colour properties and a lower content of organic residues after sintering. However, the concentrated dispersions containing oxidized alkylamines are characterized by an excessive formation of foam, which implies during the deposition process the formation of a dishomogeneous layer with presence of holes. Therefore with these dispersions, satisfactory quality coatings from the industrial point of view are not obtained.
EP 818,506 describes the use of ethoxylated alcohols as stabilizers of fluoropolymer dispersions. Said stabilizers contain from 65% to 70% by weight of ethylene oxide and have a cloud point in the range 45xc2x0 C.-85xc2x0 C. The colour and transparence properties of the obtained film result good. However, tests carried out by the Applicant (see the comparative Examples), have shown that the use of said stabilizers implies the obtainment of concentrated fluoropolymer dispersions having a limited stability to segregation and a meaningfully variation of viscosity with the temperature. High variations of the viscosity with the temperature give inconveniences during the dispersion deposition in the application step, since it is very difficult to obtain coatings having an homogeneous thickness. Besides a limited stability to segregation leads to obtain dispersions which cannot be stored for long time.
The need was therefore felt to have available aqueous concentrated dispersions of fluoropolymers able to overcome the above drawbacks of known dispersions of the prior art and having the following combination of properties:
an improved stability to segregation, in particular a remarkably more reduced segregation rate with respect to the known prior art dispersions;
a limited variation of the viscosity with the temperature, which means a better control of the dispersion deposition process for obtaining coatings having a homogeneous thickness;
high white and transparence index of the deposited film.
An object of the present invention are therefore aqueous dispersions of fluoropolymers containing as stabilizer from 1 to 30% by weight, preferably from 2.5 to 7% by weight with respect to the fluoropolymer, of a non ionic surfactant having the following structure:
CH3(CH2)nO(C3H6O)p(CH2CH2O)mH xe2x80x83xe2x80x83(I) 
wherein n, m, p are integers, and
n is in the range 8-20, preferably 12-15;
m is in the range 5-20, preferably 7-12;
is in the range 1-8, preferably it is 1 or 2.
The aqueous dispersions of fluoropolymers of the present invention have a concentration of dry polymer in the range 10%-75% by weight, preferably 50-70% by weight, under the form of colloidal fluoropolymer particles; the average diameter of the fluoropolymer particles being in the range 5-300 nm.
The lower concentration limit of the stabilizer is represented by 1% by weight based on the polymer. It has been found by the Applicant that for concentrations under said limit, the dispersion results to be not very stable mechanically. At concentrations higher than 30% by weight based on the polymer, the dispersion viscosity becomes too high for coating applications. It is preferred to use a stabilizer concentration in the range 2.5-7% by weight, and in any case such to guarantee the complete covering of the polymer particle surface in connection with their diameter.
The fluoropolymers used in the present invention are selected from:
1) homopolymers of tetrafluoroethylene (TFE), or its copolymers with one or more monomers containing at least one unsaturation of ethylene type in an amount from 0 up to 3% by moles, preferably from 0.01 to 1% by moles;
2) thermoplastic copolymers of TFE with one or more monomers containing at least one unsaturation of ethylene type;
3) homopolymers of chlorotrifluoroethylene (CTFE), or its thermoplastic copolymers with one or more monomers containing at least one unsaturation of ethylene type;
4) homopolymers of vinylidene fluoride (VDF), or its thermoplastic copolymers with one or more monomers containing at least one unsaturation of ethylene type.
The comonomers having ethylene unsaturation which can be used are both of hydrogenated and fluorinated type.
Among the hydrogenated comonomers we can mention: ethylene, propylene, acrylic monomers, for example methylmethacrylate, (meth)acrylic acid, butylacrylate, hydroxyethylhexylacrylate, styrene monomers, such as for example styrene.
Among the fluorinated comonomers we can mention:
C3-C8 perfluoroolefins, such as hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, perfluoroalkylethylene CH2xe2x95x90CHxe2x80x94Rf, wherein Rf is a C1-C6 perfluoroalkyl;
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE);
CF2xe2x95x90CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
CF2xe2x95x90CFOX (per)fluoro-oxyalkylvinylethers, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluorooxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl; fluorodioxoles, preferably perfluorodioxoles.
The TFE thermoplastic copolymers are preferably:
TFE copolymers containing from 7 to 27% by weight of hexafluoropropene;
copolymers containing from 0.5 to 18% by weight, in particular from 2 to 10% by weight of one or more perfluoroalkylvinylethers and/or fluorinated dioxoles, preferably selected from methyl-, ethyl-, propylvinylether, 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TDD), perfluoro-2,2-dimethyl-1,3-dioxole (PDD).
The dispersion of the present invention can be obtained starting from the polymerization latex by addition of the non ionic surfactant having structure (I). To obtain a concentrated dispersion, after the addition of the stabilizer, the dispersion concentration can be carried out. Concerning this, one of the following techniques can be used: evaporation under vacuum, electrodecantation, ultrfiltration.
As regards the polymerization of fluoropolymers, it can be carried out in aqueous emulsion according to methods well known in the prior art, in the presence of radical initiators, both of inorganic type (alkaline or ammonium persulphates, perphosphates, perborates or percarbonates), and of organic type (peroxydisuccinic acid), optionally in combination with ferrous, cupric or silver salts, or of other easily oxidizable metals. In the rection medium fluorinatd ionic surfactants of formula:
Rfxe2x80x94Xxe2x88x92M+
are present, wherein Rf is a C5-C16 (per)fluoroalkyl chain or a (per)fluoropolyoxyalkylene chain, Xxe2x88x92 is xe2x80x94COO or xe2x80x94SO3xe2x88x92, M+ is selected from: H+, NH4+, an alkaline metal ion. Among the most commonly used we remember: ammonium perfluorooctanoate, (per)fluoropolyoxyalkylenes ended with one or more carboxylic groups, etc.
Alternatively, the polymerization is carried out in the presence of a microemulsion of perfluoropolyoxyalkylenes, according to U.S. Pat. No. 4,789,717 and U.S. Pat. No. 4,864,006.
The polymerization reaction is generally carried out at temperatures in the range 25xc2x0-150xc2x0 C., under a pressure up to 10 MPa.
Without to be bound to any theory, the Applicant keeps that the particular structure of the surfactant of the invention increases the stabilizing power of the dispersion, and in particular, that by increasing the hydrated radius of the colloidal polymer particles, the segregation rate decreases. Besides, the particular structure of the surfactant of the invention positively affects the rheological behaviour of the concentrated dispersion, since the viscosity shows a limited variation in function of the temperature in the range 20xc2x0-40xc2x0 C., commonly used for coating applications.
The concentrated dispersion of fluoropolymers of the present invention shows an improved stability to segregation, since it shows a segregation rate lower than about 50% (see the Examples) with respect to a dispersion stabilized with polyethoxylated octylphenol (commercially known as TRITON(copyright) X100) used in the preparation of the PTFE-based dispersions. Besides, the dispersion of the invention is characterized by a limited viscosity variation with the temperature, wherefore it is particularly suitable to be used for obtaining homogeneous without defect coatings on substrata of various nature, even by the multilayer deposition of fluoropolymers. Besides the stabilizers of the invention do not give colouring problems.
The dispersion of fluoropolymers of the present invention can be used as such or in specific formulations for the coating of metal, ceramic supports or in the impregnation of fibers and textiles of various materials. As regards the deposition, dipping, casting techniques, roll, film, or spraying application can be used.
The dispersion of the present invention can optionally contain the following components in the amounts known in the prior art:
water, organic solvents, such as for example, ethyl or isopropyl alcohol;
adhesion promoters, dyes, pigments;
metal oxides for improving the coating mechanical properties;
salts, for example potassium chlorate having the purpose to improve the dispersion filmability,
Further fragrance substances, e.g. pinene derivatives, e.g. xcex2-pinene polyethoxylated can be added to confer a pleasant smell. The amount of fragrance substances is e.g. in the range based on PTFE+water of the dispersion, 0,05-5% by wt.
Besides, to improve the wetting capability of the dispersion of the invention the following components can be optionally added, in an amount preferably in the range 500-3,000 ppm with respect to the total dispersion (water+fluoropolymer):
anionic, preferably fluorinated, surfactants such for example ammonium perfluorooctanoate, compounds of structure Fxe2x80x94(CF2xe2x80x94CF2)nxe2x80x94CH2xe2x80x94CH2xe2x80x94SO3M (FORAFAC(copyright) 1033D), ClC3F6O(C3F6O)nxe2x80x94CF2COONH4 (FLUOROLINK(copyright) 7004);
fluorinated non ionic surfactants, such for example the compounds of structure CF3(CF2)5xe2x80x94(CH2CH2O)8H (Forafac (copyright) 1110D), C3F6Clxe2x80x94Oxe2x80x94(CF2CF(CF3)O)m(CF2O)nxe2x80x94CF2xe2x80x94CONHxe2x80x94(CH2CH2O)21CH2CH2OCH3 (Fluorolink 7005(copyright)).
The dispersions of the invention show very good properties of stability to segregation and application characteristics such as to allow the application of subsequent layers of fluoropolymer, even with high productivity techniques, such as impregnation, casting and dipping, without the appearance of defects such as cissings, coagula or foam.
The present invention will now be better illustrated by the following embodyment Examples, which have a merely indicative but not limitative purpose for the scope of the invention itself.